In numerous technological fields high-pressure hydraulic systems have to perform, via setting means, switching operations which directly affect the pressure medium. Because of the high pressure, it is, as a rule, not possible to directly control the setting means and therefore a servocontrol circuit has to be provided which switches the setting means via a hydraulic control valve with a small part of the pressure fluid acting as a servo flow, whereby very short switching times may be obtained.
An accurate control of the servoflow is indispensable for a good reproducibility of the operation of the setting means. A particularly superior control is obtained if the hydraulic control valve is pressure-equalized. Such control valves are known as plunger valves or plunger seat valves. It is a disadvantage of the conventional plunger valves that a relatively long sealing portion on the plunger body has to be provided between the control edges of the valve. For this reason, plunger valves which are to be operated only with small valve strokes of, for example, 50 .mu.m, cannot be used in a high-pressure environment where the pressure is 1000 bar and more.
Instead of plunger valves, plunger seat valves may be used which do not have the above-outlined disadvantage because the supply and/or removal of fluid concerning the pressurized fluid to be controlled is sealed in the corresponding switching states by one or more valve seats in the valve chamber. A disadvantage of known plunger seat valves, however, is seen in their high manufacturing costs, particularly those involved with the required grinding and polishing steps.
German Patent document 27 56 008 describes an electrically switchable fuel valve which includes a plunger component composed of a plurality of individual bodies. The individual bodies are held together by two oppositely directed springs since the individual bodies are not guided separately. Such a valve, however, is usable only for low pressures and is not pressure-equalized with respect to the plunger part.
U.S. Pat. No. 4,628,881 describes a pressure-amplifying fuel injector which comprises two valve balls. The fuel which is initially pressurized (for example, to 100 bar) is amplified by means of a stepped piston to a high injection pressure of approximately 1500 bar. The charging of the driving piston is effected by means of a 3/2-way valve. A drainage chamber is positioned directly behind the second valve ball, and thus the valve cannot be pressure-equalized at any time. For opening the valve, the electromagnet has to displace the valve balls against the large forces derived from the preliminarily pressurized fluid. The connecting rod between the individual valve components is not guided in the region of the valve seats, because at that region the fuel must pass through the valve. The valve balls are held together only by the preliminary pressure of the fuel; no further spring is provided. Such a construction does not ensure in all operating points a safe, reliable operation of the valve. At least at operation start, that is, upon build-up of the preliminary pressure, the valve needs time until the first valve ball finds its seat and seals. In the open position of the valve the first valve ball is exposed to the preliminary pressure at all sides and thus it floats in the fuel stream substantially without force effect and may thus be separated from the connecting bar.